This invention relates to the control of gas turbine engines and helicopter rotors.
Generally, the helicopter has two primary control means, namely the pitch angle (angle of attack) of each main rotor blade and the pitch angle of the tail rotor assembly. The former effectively provides control over all vertical and lateral directions as well as forward and backward motion; the latter, apart from reacting the main rotor torque also provides control over the fuselage heading.
Concentrating on the main rotor control, each blade has an individual pitch angle set by the aircraft flying controls. If all blade angles are changed together, the aircraft will move up or down. If the blades at the rear are changed relative to those at the front, fore and aft motion of the helicopter is controlled. If the blades at one side are changed relative to those at the other, lateral motion is controlled.
The main rotor pitch control mechanism is clearly very complex, requiring high power servo actuators and sophisticated engineering. The pitch controls are positioned in accordance with the pilot demands, and he is provided with two cockpit levers for these basic control purposes. The levers are, the collective pitch lever, which moves the blades collectively to achieve up and down flight control, and the cyclic pitch lever which moves the blades cyclically during any one revolution of the rotor and thereby provides fore/aft/lateral control. The pilot also has some control over tail rotor pitch, via foot operated rudder pedals for aircraft yaw control.
The mixing together of pilot's inputs, to achieve a stable and predictable aircraft flight path, is a major helicopter design problem. The movements of each flying control interact with the others, so that, for example, a demand for a left turn can cause some loss of height without an equivalent collective pitch compensation.
Helicopters, such as the Westland Helicopters PLC Lynx, make an already difficult situation worse by adopting a rotor design configuration, with rigid or semi-rigid rotors which are all but unusable except with artificial stability augmentation systems. Even with these aids, such helicopters remain extremely difficult to fly, and pilots have to be trained to observe many limitations when attempting to maneuver such as, rates of change of collective pitch, turn rates, combinations of collective pitch and cyclic pitch, etc. This situation comes into focus with the new generation of light military helicopters which require not only a fast (agile) response from the aircraft but also a reduced pilot workload to fly them due to the role expected from the helicopter.
In order to overcome the above mentioned problems, a new generation automatic flying control system (AFCS) is being developed by a number of aircraft manufacturers. This system, known as Active Control Technology (ACT), is not the subject of this patent application and is therefore not described in detail herein; however, it is sufficient to say that any direct coupling of the pilot's controls to the rotor system is replaced by a ccmputerised arrangement of sensors, digital intelligence and electromechanical actuators. In time, it is expected that ACT will be capable of sensing the pilot's demands and determining the best way of meeting those demands while automatically taking into account the limitations that the pilot previously had to observe, thereby providing a carefree handling helicopter.
A further aspect of the control problem, outside the influence of ACT, is the rotor speed itself; the lift developed by each blade depends not only on its angle of attack but also its velocity relative to the air. Rotor speed control is therefore important in overall rotor performance and hence critical to aircraft performance and controlability.
Many modern helicopters use free turbine turboshaft engines to provide power to the rotor. In such helicopters, the rotor is directly driven from, and is therefore directly dependant upon, the speed of rotation of the free turbine. In order to control the speed of rotation of the rotor, it is common to provide a free turbine governing system. This system senses the speed changes resulting from movement of the rotor blades and responds by changing engine power to maintain free turbine (rotor) speed to within desired limits. The accuracy of the governing system is critical in maintaining the desired level of torque matching between engines in a multi-engine application. The response of the whole engine/governing system is critical in preventing excessive transient variations in rotor speed, and special means are required to ensure torsional stability. The interface between the rotor system and the engine/governing system is a complex one and has given rise to many problems.
These problems are centered-around the necessity for the helicopter manufacturer to delegate the responsibility for controlling the speed of the lift generating surfaces (the rotor) to the engine manufacturer, thereby only having an indirect influence on a prime aircraft performance parameter.
It is clear that, while both Active Control Technology and engine governing techniques offer a great improvement in the performance of a helicopter, neither is capable of achieving optimum control over the helicopter since each is lacking in at least one control aspect.